(1) Field of the Invention
This invention relates to paper making and paper printing, and is directed more particularly to a paper forming fabric for making paper better suited for receiving print thereon, and to a method for producing a printed piece of paper utilizing the paper forming fabric.
(2) Description of the Prior Art
It is known that the quality of print on paper is directly related to the uniformity of the density of the sheet of paper on which the printing ink is deposited. High density paper results in low ink penetration and high gloss of the printed product, while low density paper results in greater penetration and low gloss in the appearance of the print.
The weave pattern of paper forming fabrics results in "knuckles" wherein yarns cross each other and provide raised yarn portions in the surface of the fabric. Such "knuckles" are disposed at an angle to the width of the forming fabric, the angle often referred to as the "twill angle" of the fabric.
Such woven structures create micro density differences in a sheet of paper formed thereon, with relatively low density occurring over the knuckles of the fabric and relatively high density occurring over the holes bounded by the yarns of the fabric. Such low and high density areas, alternating along a line corresponding to the fabric twill angle, produce areas of different print quality along the line because of the different degrees of ink penetration into the sheet.
In color printing anything other than solid colors, that is, in printing images of people, places or things, using shading, or the like, printers use printing screens, with each screen laying down one of four basic colors and having micro droplets of ink, or "dots" applied to the paper at an angle different from the angle at which the micro droplets of the remaining screens are applied to the paper. The standard angles at which the basic color ink droplets are applied are 45.degree. for black, 75.degree. for magenta, 90.degree. for yellow, and 105.degree. for cyan. Each color is laid down at a different pre-determined angle in order to prevent "moire" which is a term given to describe overlay, or overlapping of colors.
The printing screens themselves typically are of copper and are acid-etched to provide millions of micro surfaces for retention of the aforesaid ink droplets. The droplets are arranged in diagonal rows at the aforesaid prescribed angles for basic colors. Such angled rows of droplets are referred to as "diagonals". Study has revealed that when the fabric twill angle produces a line of alternating high and low density areas in paper, and the diagonals of one or more of the printing screens coincide with that line on the paper, the coincidence of such density difference in the paper and the diagonals of the printing screens can create an adverse variation in the uniformity of the print quality due to a moire' effect between the twill lines of the forming fabric and the diagonals of any of the basic print screens.
It therefore is beneficial to have the diagonals of the print screens not coincide with the line of high and low density areas in the paper generated by the forming fabric twill angle.